Of course Tolvo makes a great one. I demand Tolvo to be in Dark Souls 2.
Anyways, I have multiple characters so I will tell the stories of them if they were NPCs.
Mary of Berenike
She would first be found standing on the large branch leading up to the Great Hollow tree, wearing the full warrior set with a claymore and tower shield.
First dialogue: Hello. You seem to not be trying to kill me at the moment. Wouldn't happen to have any heavy armor on you, would you?
If yes: Great. I would gladly take for you.......Oh, but that wouldn't be fair. I will pay you for this.
If no: Oh......well, if you have anything else, I would gladly be willing to pay for them.
Dialogue after the first encounter: Any more treasures you find in your travels?
Upon leaving her shop without selling anything: None eh? See you when you find some.
Upon leaving her shop after selling something: More for my stash. Thank you.
She would be a merchant that you could sell her anything you have, similar to Frampt. Except that later you could come back and try to buy it back from her, but with it being 20 to 30% higher priced then what you sold it for. And any heavy armor you give her, she will put on and wear until she finds armor with better physical protection.
Upon being attacked: You are no different from the others. Another thing in my way. Today is the day you die.
Upon victory over the player: Oh dear, I hope Hector does not find this body. He will most certainly know it was my doing....
Upon being slain by the player: Huh.....You have bested me......Hector, please......forgive me....
She will also randomly switch between using magic and pyromancy. Will be incredibly aggresive.
Her second encounter would be in Anor Londo, in the top area of the church, near where the Dragon Slayer bow is found.
First dialogue: He is near.....I know this.....He mustn't see me like this....
Second dialogue: Hm? Ah! My apologies. Just.....thinking. Find more things while you were gone? I certainly did.
At this point she will sell some minor pyromancies and sorceries. Other then that her dialogue will be the same here.
Last encounter. She will be in the Kiln, between the first two black knights, and will be hollowed. She will attack the player on sight. Her back is turned, however, and if the player gets close without her noticing, they will hear her speaking madness.
Dialogue when she does not see the player yet: Gwyn......I must kill Gwyn. I have killed so many already. So many......They were just in my way.....only in my way.....If I just kill Gwyn.....Kill.....everything.....
She will repeat this until noticing the player.
Dialogue when seeing the player: Hector....? Hector! No! Do not see my like this! You must not! Must not! Die!
She will begin to use Chaos pyromancies, dark magic, and crystal sorceries. She will also switch her weapons randomly, specifically any weapon that has a special R2, but only any weapons that was given to her by the player.
Upon death: Heh.....Hahaha.......HAHAHAHA!......I tried so hard to survive......and only with death....I find peace....Thank you.
Upon killing the player: Die! DIE DIE DIE DIE! Come back and I will kill you again! Like everyone else!
Will drop whatever weapons she is holding at the moment.
Hector of Balder
Will be standing in the tower in undead Burg, on the floor just before the Taurus Demon. He will be wearing full giant armor except for the helm, holding a Greataxe and Stone Greatshield. He will immediately address the player once he sees them.
First dialogue upon seeing the player: Hey! You! Stop where you are, a knight is addressing you. Come hither.
If the player walks by him: You will regret that, knave.
If the player talks to him: Now, have you seen a woman named......Mary? I happen to be looking for her.
If no: Bah, begone with you now. Come to me once you see her.
If yes: You have? Splendid. I knew she was here. I suppose I should award you for that information........Here.
He will then give you a single Llyod's Talisman.
If attacked: Pitiful fool. If you wish to see how a true man fights, then so be it. Prepare to die!
Upon victory over the player: Feh, I knew you would not put up a challenge. Now, to go looking.....
Upon death: AAARRRGGHH!!! This cannot be! I will kill you! I will claw my way out of the clutches of death, and come back to kill you, you wretched undead!
Will fight using many miracles, and at this point will only use force, replenishment, great heal, and emit force.
Second encounter: He will be in the Painted World, on top of the build with the dried finger in it. He will be looking off over the ledge.
First dialogue: What? You again? Your advice helped none! I have been everywhere and have yet to find her. I will have to use more creative methods to find her......Yes, indeed.
Second dialogue: If you are thinking of kicking me off the ledge, I would not advise it, undead. I have a new trick I could show you that would make that highly inadvisable......
If he is fought right then, he will now also use the miracle Wrath of the Gods, and seems to have infinite casts of it.
At the end of the Painted World, where you find Jeremiah's armor, you will also find a black eye orb, with the following description: "An orb of uncertain power, and left in the Painted World for unknown reasons. Where could the owner of this orb be?"
Third encounter: The black eye orb with start to quiver when you get to the Kiln, and when using it, will invade the world of Hector. He will be wearing the Mask of the Mother, and be screaming at you. He will put Sunlight Blade on his Greataxe as soon as he sees you.
Dialogue as the fight starts: WHERE IS SHE! Where is Mary!? I know she is here! I know it! I will kill every undead there is until I find her!
Dialogue upon hitting the player: You know where she is don't you? DON'T YOU!?
Dialogue upon victory over the player: I will find her......You and every undead will fall to my blade.
Upon death: No! I must.....Find her.....Mary.....I am sorry.....You are all I have....
Will drop 10 humanity, a canvas talisman, his shield, and Priscilla's Dagger.
Alvaro
Can first be found in the tower where the second Black Knight is, the one with the Ultra Greatsword. He will spawn there after the black knight is killed. He will be wearing his own armor set, a large grey cloak that covers most of his body, a hood that makes a shadow over half his face so that you only see his smile, black gloves and leggings with many different daggers and knives strapped to his legs.
First dialogue: Look, a new friend. My name is Alvaro, and yours.....? Never mind. Hm, let me take a guess, chosen undead, escape from the asylum, prophecy and what not telling you to......Well I won't spoil that part for you. I was once like you. Once......Hahahahaha!
Second dialogue: You seem halfway decent, so tell you what. I will help you. On one condition.......You give me something......
You will then be prompted to give him the copper coin you get from Petrus
If yes: Yes, yes, this is very nice......Okay, my blades are at your command. So long now, "chosen undead".
If no, or if you do not have it: Drat! You are no fun! Okay, I will give you one last chance......What I will request is a special item, very rare.....Do you have it?
He will then request for Rubbish.
If yes: Yes, yes, this is very nice......Okay, my blades are at your command. So long now, "chosen undead".
If no: You are pretty poor aren't you? Hmm......I'll help you anyways. Now go along now, before I change my mind. So long, "chosen undead".
He will then be available for summon at the Gaping Dragon and Bell Gargoyles. He will be using the Dark Wood Grain Ring with a parrying dagger and normal dagger.
If attacked: You wish to challenge me? How fun! Let us see the strength of your blades!
Upon victory over the player: It appears you underestimated me. Like all the others. Sad.
Upon death: Huh.....I am dying......This I did not expect......So long now, you have earned this victory.....
Second encounter. He will be standing at Firelink Shrine, to the left of where Seigmeyer is, after the player has acquired the Lord Vessel. He will be sitting down, and sound like he is muttering to himself, though the words are not clear enough to be understood.
First dialogue: You.....you are a good fighter. Just like I am. And also like I am. Maybe I should help you.....Maybe. But I need.....I need humanity. He tells me I need it. I need so much of it apparently.....
Second dialogue: You wouldn't happen to have met a man in shiny bronze armor, would you? Lautrec? He seems to be one of the only people in the right of mind. He knows what to do. You should listen to him.
Third dialogue: I met a hyena today. He told me how to get free things. There are a few skeletons in the way......But all I need to do is pull a lever here, and off they go! Free things! Such a smart man.
Fourth dialogue and any dialogue after: Why, my friend, I suggest you back away from me. I might stick something sharp into you......And not in the fun way.
Third encounter: When entering the Crystal caves Alvaro will invade. He will shoot poison arrows, poison throwing knives, and switch to the Darksword randomly. He will also throw Llyod's talismans.
Dialogue upon seeing the player: We will kill you, friend. We will win. He tells me we will win. You are not the chosen undead, we are!
Dialogue during battle: Lalala la laaalala! - Hacking and slashing and blood and death! - Your humanity is ours! OURS! - I am the one destined to kill the lords, I am the one! We will do it, we will!
Upon getting a headshot with an arrow: Boom!
Upon victory over the player: Of course it ended this way. It always was destined to end this way. They tell me this......him and the serpent....I am sorry.
Upon death: But.....they told me, they told me I would win......Congrats, friend......You have bested me.